This invention relates to an ultrasonic transducer with a multiple-folded piezoelectric polymer film.
In general, as a linear array type ultrasonic transducer for use on a linear electron scanning system use may be made of an array type in which a ceramics piezoelectric substratum, such as lead titanate or lead titanate zirconate, includes strip-like elements. This type of ceramics piezoelectric substratum is hard and brittle in nature and tends to produce defects and cracks when the strip-like elements are obtained. Furthermore, it is difficult to precisely form many strip-like elements. Many problems are also involved from the standpoint of manufacturing costs.
It is known that a fluorine-containing high polymer, such as polyvinylidene fluoride (PVF.sub.2) or polyvinylidene-triethylene fluoride copolymer (PVF.sub.2.TrFE), or the other organic synthetic high polymer is polarized at high temperatures under a high electric field to manifest its piezoelectricity and pyroelectricity. Recently, an ultrasonic transducer has actively been developed utilizing the thickness shear mode of the piezoelectric high polymer. The specific acoustic impedance of this piezoelectric polymer is close to that of a human body and, moreover, a smaller elasticity is involved on the piezoelectric polymer. It is said that, if the piezoelectric polymer is applied to a linear array type ultrasonic transducer, it is unnecessary, unlike the ceramics piezoelectric substratum, to obtain strip-like elements by a cutting operation or a separating operation.
The dielectric constant of the piezoelectric polymer film is, in general, of the order of 10, i.e., prominently smaller than that of the ceramics piezoelectric substratum. Furthermore, the drive elements of the linear array type ultrasonic transducer have a smaller area and an extremely high acoustic impedance. Usually, a poor matching is involved against a 50 .OMEGA. power source (transmitting/receiving circuit), suffering an appreciable loss on the ultrasonic transducer.
In order to solve the above-mentioned problems, an ultrasonic transducer has been proposed in which a plurality of piezoelectric polymer films are properly piled up to obtain a thicker polymer film while at the same time the electric impedance is lowered. This type of conventional ultrasonic transducer is shown in FIG. 1. In the conventional ultrasonic transducer, a plurality of piezoelectric polymer films (3, 3, 3), each, have strip-like electrodes 1 on one surface and a common electrode 2 on the other surface and are piled up such that the two adjacent piezoelectric polymer films have their identical electrodes located opposite to each other as shown in FIG. 1. The opposite, identical electrodes of the adjacent two polymer films are connected by a solder or a conductive adhesive 4 to each other. For example, the strip-like electrode 1 of the first piezoelectric polymer film is located opposite to the strip-like electrode 1 of the corresponding adjacent second piezoelectric polymer film. Such a type of ultrasonic transducer is known which lowers an electric impedance. With Zo representing an electric impedance of, for example, a single layer of a resonant frequency f, EQU Z =Zo/n.sup.2
(n: the number of layers) for the ultrasonic transducer of the configuration as shown in FIG. 1. An electric impedance of 1/4 is involved for a two-layer structure and an electric impedance of 1/9 is involved for a three-layer structure. It is, therefore, possible to obtain an improved matching with respect to a power source. In the conventional arrangement as shown in FIG. 1 it would be difficult to take leads 5a and 5b out of the electrodes 1 and 2, respectively.
An ultrasonic transducer of such a type as shown in FIG. 2 has also been proposed which has a continuous, piezoelectric polymer film 3a properly folded as a multiple-layer structure of a desired thickness. In this transducer, it is easier to take leads from the corresponding electrodes and it is also possible to lower the electric impedance. However, the following problems arise therefrom.
That is, if a continuous, piezoelectric polymer film is to be folded, it would be difficult to precisely locate the corresponding areas of the strip-like electrodes opposite to each other. In this case, a possible displacement is produced in the vertical directions of the electrodes 1, causing a difference in the electric impedance of drive elements and producing a possible shorting between the drive elements. This problem becomes prominent with an increase in the number of layers so piled up.